Head mounted displays (HMDs), which are used while worn on a part of the body such as the head to place a video screen in front of each eye of the user, find applications in various fields including the field of virtual reality. Typical HMDs are designed to block out outside light and force the user to see only the HMD's pictures. It usually has a shape of a frame of goggles or large glasses. With an HMD used, the user can view images irrespective of the direction he or she faces but then again it gives the user visual isolation from the surrounding environment.
In recent years, small image display devices have been proposed that allow users to view images as well as the surrounding environment by means of presenting the images in a certain part of the field of vision.
The image display device of this type is disclosed in, for example, Japanese Patent Laid-open No. 7-209600. The image display device has a liquid crystal display (LCD), a reflecting mirror, an ocular lens and other components contained within a single housing. The housing has a clip attached thereto. This clip is used for mounting the image display device onto a frame of glasses or sunglasses. The housing is placed on top of the frame of the glasses over the outside surface of it. An image that is displayed on the LCD is directed to one eye of the user in a diagonal down direction through the reflecting mirror, the ocular lens, and the lens of the glasses.
The image display device of this type provides the user with a choice between seeing the surrounding environment and seeing images by the movement of the eyes. It has a high value in terms of its availability in everyday life at any place both inside and outside a building.
Accordingly, such an image display device may be used as a visual version of portable stereo headphones through which sound comes out and which has significantly changed the lifestyle of young people.
The image display device of this type is intended to be attached to existing glasses. In this case, the user manually attaches the image display device to the glasses himself or herself. However, positioning of the image display device is important in order to allow the user to view images. It is not so easy to properly position the image display device and attach it to the glasses.
In addition, the image display device as described above is expected to be often attached to and removed from the glasses because the user removes the image display device from the glasses when he or she doesn't want to use it, and attaches it to the glasses when he or she wants to use it. The difficulty in exact positioning of the image display device for the attachment to the glasses could be a cause of users' steering clear of using the image display device.
The present invention is for solving the aforementioned problem, and an object thereof is to provide a technique with which a user can use a head mounted display device without any difficulty in positioning of it.